The present invention relates to a distributor for use in an electronic ignition system for an internal combustion engine such as gasoline engine, and more particularly to a distributor suitable for use in a vehicle engine.
The ignition system of an internal combustion engine, such as a gasoline engine of a vehicle, generates radio frequency noise due to sparking between the spark plug and the distributor. Since the radio noise has a broad frequency band, there is a strong likelihood of resulting interference to various radio communication systems, or a malfunction in various electronic devices mounted on the vehicle.
As shown in FIG. 3, referred to hereinafter, a distributor of the type described above generally has a housing 18 and a camshaft 19 which rotates in fixed relation to the rotation of the internal combustion engine. A distributor rotor arm 12 attached to the top of the camshaft 19 has a rotating electrode 10 on its insulating top surface, and stationary electrodes 16 are provided facing toward a sparking surface 2 in the rotating electrode 10. The plurality of stationary electrodes 16 are positioned along the periphery of the rotation locus of the rotating electrode 10.
A center terminal 14 situated above the rotation center of the rotating electrode 10 is connected to it through a spring 15 and a carbon point 13. The stationary electrodes 16 and the center terminal 14 are contained in a distributor cap 17 mounted the housing 18.
In a distributor having the structure described above, when the rotating electrode 10 comes to a position facing one of the stationary electrodes 16, the high voltage generated by a primary current switching device in the ignition system is conducted through the rotating electrode 10, the center terminal 14, the spring 15 and the carbon point 13. It is then transferred to the stationary electrode 16 by an electric breakdown of the air in a small gap 11 between the sparking surface 2 in the rotating electrode 10 and the stationary electrode 16, to supply a specified spark plug.
The spark between the rotating electrode 10 and the stationary electrode 16 constitutes a source of radio noise. Therefore, such an ignition system has heretofore been provided with various means for suppressing the generation of the radio noise. For example, one method which has widely been used and recognized is to provide a resistor spark plug and a resistor high voltage lead wire.
It has also been proposed to add a dielectric material onto the sparking surface of the rotating electrode in a distributor, which has the effect of lowering the ignition voltage at the start of discharge. For example Japanese Patent Application Laid-Open No. 53-90536 (1978) discloses a method in which a dielectric member is provided projecting on a rotating electrode, and Japanese Patent Application Laid-Open No. 59-226278 (1984) describes a method where silicone varnish is painted on both top and bottom surfaces of a rotating electrode. Japanese Patent Application Laid-Open No. 61-76764 (1986) describes a method in which a dielectric member is adhered closely to a rotating electrode by using a metallic mesh, and in Japanese Patent Application Laid-Open No. 61-53461 (1986) metallic oxide is thermally sprayed onto a rotating electrode.
Each of the above described noise suppression techniques suffers from the same disadvantage: in particular, they make it necessary to increase the sparking voltage, and heat from the spark over a long period of time tends to cause the materials to deteriorate and break down in the area adjacent to the sparking face of the rotating electrode, so that the noise suppression effect is impaired.